NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have already put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, selective catalytic reduction (SCR), and lean NOx traps (NOx adsorber-catalysts).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn catalysts are unacceptably low.
SCR refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
NOx adsorber-catalysts are NOx adsorbers combined with catalysts for NOx reduction. The adsorbant is typically an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst is typically a precious metal, such as Pt or Ru. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. Accumulated NOx is removed by creating a rich environment within the NOx adsorber-catalyst through the introduction of a reductant. In a rich environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed, preferably as N2. The process of removing accumulated NOx from the NOx adsorber-catalyst is commonly referred to as regeneration, although it may also be referred to as denitration in order to distinguish desulfation, described below.
In addition to accumulating NOx, NOx adsorber-catalysts accumulate SOx. SOx is the combustion product of sulfur present in ordinarily diesel fuel. Even with reduced sulfur fuels, the amount of SOx produced by diesel combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere.
The conditions for denitration may be created in several ways. One approach uses the engine to create a rich fuel-air mixture. This may be accomplished, for example, by injecting extra diesel fuel into one or more engine cylinders after combustion and substantial decompression. Reductant may also be injected into the exhaust downstream of the engine. In either case, a portion of the reductant must be expended to consume oxygen in the exhaust. The reductant can consume oxygen either by reactions in the NOx adsorber-catalyst or by reactions in an upstream unit. For example, U.S. Patent Pub. No. 2003/0101713 describes an exhaust system with a fuel reformer placed inline with the exhaust and upstream of a NOx adsorber-catalyst. The fuel reformer not only consumes excess oxygen, but converts diesel fuel into more reactive reformate.
It is known that a NOx adsorber-catalyst can produce ammonia during denitration and from this knowledge it has been proposed to combine a NOx adsorber-catalyst and a SCR catalyst into one system. Ammonia produced by the NOx adsorber-catalyst during regeneration is captured in a downstream SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone NOx adsorber-catalyst with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes such a system. U.S. Patent Pub. No. 2004/0076565 describes such a system wherein both components are encased by a single shell or disbursed over one substrate. WO 2004/090296 describes such a system with an inline reformer upstream of the NOx adsorber-catalyst,
U.S. Patent Pub. No. 2004/0101713 suggests that an inline reformer can overheat a downstream NOx adsorber-catalyst. Accordingly, that publication proposes a device providing a thermal mass placed between the reformer and the NOx adsorber-catalyst to prevent such overheating. The device can be a particulate filter.
U.S. Patent Pub. No. 2004/0050037 proposes a method of regulating the temperature of a NOx adsorber-catalyst downstream of a reformer during desulfation. According to this method, a reductant flow is pulsed. In between pulses, the NOx adsorber-catalyst is allowed to cool to prevent overheating.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost, and can practically be used to reduce NOx emissions across the spectrum of diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.